1. Field of the Invention
The invention relates to a tracking control device and a tracking control method for converging a main beam and a sub beam on an information recording medium having plural information layers, and controlling a convergence point of the main beam to scan a track on the information layer. The invention also relates to an optical disc apparatus for optically recording information on an information recording medium or optically reproducing information from an information recording medium.
2. Description of the Background Art
Conventionally, there have been proposed various methods for detecting a tracking error signal to be used in tracking control in an optical disc apparatus. An example of the methods for detecting a tracking error signal is a differential push-pull method (hereinafter, also called as a “DPP method”).
As is already known, in the DPP method, a light beam to be irradiated onto an optical disc is divided into three beams constituted of one main beam and two sub beams, and a push-pull signal is detected with respect to each spots of the main beam and the sub beams. Then, a differential signal between the push-pull signal of the main beam, and the push-pull signal of the sub beam is detected as a tracking error signal. In the DPP method, an offset is generated in both of the push-pull signal (hereinafter, also called as an “MPP signal”) of the main beam, and the push-pull signal (hereinafter, also called as an “SPP signal”) of the sub beam by e.g. lens shift or lens tilt. The offset can be cancelled based on a differential signal by properly setting a ratio between the MPP signal and the SPP signal (see e.g. JP Sho 61-94246A and JP 2005-310257A).
In the conventional optical disc apparatus, however, it is generally often the case that the intensity ratio between the main beam and the sub beam is set to about 10:1. In the case where information is recorded or reproduced on or from an optical disc having plural information layers such as a two-layer disc by the optical disc apparatus having the above configuration, the following drawback may occur.
Specifically, in the case where a light beam is converged on one of the information layers in a two-layer disc, it is difficult to separate reflection light from the one information layer and reflection light (stray light from the other layer) from the other of the information layers. As a result, a stray light component of the main beam from the other layer may be detected by a sub detector for receiving a sub beam. In particular, in the case where information is recorded without high frequency superimposition, an SPP signal may be fluctuated by interference of stray light of the main beam from the other layer. As a result, tracking control may become unstable, and in a worst case, a light beam may be off the track at the time of recording.